If this page seems to have an American cultural focus, it's only because the few people who have contributed to this list are -- ta da! -- Americans. PLEASE feel free to add idioms, humor, imagery, etc. for any culture you are familiar with (with a preference towards providing English translations, since the site is predominantly English)!
There are everyday images, sayings and idioms that point to the possibility that more people are multiple than we might think. Whether this is an effect of multiplicty being pervasive already, or whether these give a message to young people that multiplicity is a viable option is arguable. Regardless, this is evidence that such ideas are in fact out there in common use. This list includes some of them.
"I lost my head!"
"I couldn't live with myself...!"
"I was a different person back then!"
"That's it! No more Mr. Nice Guy!"
"I had an argument with myself over..."
"That wasn't like me" (or "that wasn't like you")
"I'm of two minds on the issue..."
"Pat myself on the back..."
"I'm beside myself with worry..."
"What's gotten into you?" (for all those walk-ins, channelers, and hosts out there)
"I need to make up my mind"
"Pull yourself together..."
"I don't feel quite like myself today."
"...put on a good face..."
"I'm coming apart at the seams..."
"A part of me died..."
"My mind wandered..."
"...she fell apart..."
"...my inner child..."
"...get my act together..."
We see it all the time in cartoons, where on one shoulder there's the angelic "good conscience" and on the other is the demonic "bad conscience" who often converse with the "self", even to the point of beating on each other, taking one another out of commission, tricking each other or the self, etc. (strong Freudian imagery of Id, Superego and Ego here)
A creative version on the theme:
www.nald.ca/CLR/faraway/ stories/s2/images/6-2.jpg (Where "conscience" is defined as "Conscience : The part of your mind that tells you if a thing is right or wrong." -- note the definition's use of "part" and compare that with "parts" in Self Plurality)
Cartoons where one's shadow or mirror image takes on a life of it's own, often doing things that one might not approve of on one's behalf, before one can stop it. (strong Jungian imagery)